1. Field of the Invention
The present invention relates to a capacitor element and a method for producing the same in the case where a ferroelectric thin film is formed by a reaction rate-determining method. It particularly relates to a capacitor element capable of providing a ferroelectric memory (hereinafter referred to as FeRAM) that has smaller variation in characteristics on a lower electrode with a thickness of not more than 100 nm, and to a method for producing the same.
2. Related Background Art
A FeRAM is a non-volatile memory characterized by the high-speed rewriting and the increased writing times. The miniaturization of a memory cell thereof, particularly the miniaturization of a capacitor element composing a memory cell in a longitudinal direction (indicative of a thickness direction) and a transversal direction (indicative of a horizontal direction perpendicular to the thickness direction), has been demanded with the advancement of the integration. For the miniaturization, as to the longitudinal direction (thickness direction), it is necessary to make thinner a lower electrode, a ferroelectric thin film, and an upper electrode that compose a capacitor element. As to the transversal direction (horizontal direction), it is necessary to form the capacitor element three-dimensionally.
Thus, whether in the longitudinal direction or in the transversal direction, it is necessary to form films thinner. Therefore, for forming a ferroelectric thin film, it is essential to use a reaction rate-determining method with which the film thickness can be controlled so that the thickness decreases, for instance, the metal organic chemical vapor deposition (MOCVD) method. For instance, a method for producing a capacitor element for use in a DRAM in which a lower electrode (Ru) and a ferroelectric thin film (BST, i.e., (Ba, Sr)TiO3) are formed by MOCVD has been disclosed (WO 00/22658).
However, the following phenomenon was found to occur in the case where a lower electrode is thinned so as to be not more than 100 nm in thickness: if a ferroelectric thin film is formed by a surface reaction rate-determining method such as MOCVD, the composition of the ferroelectric thin film varies with the thickness, due to the diffusion to the lower electrode of metal atoms that form the ferroelectric thin film.
To show an example of the same, the graph of FIG. 8 shows the film-thickness dependency of SBT (SrBi2Ta2O9) formed as the ferroelectric thin film by MOCVD with respect to the film thickness of a Pt lower electrode, that is, the variation of the composition of SBT with respect to the thickness of the lower electrode. In FIG. 8, a solid circle indicates the Bi composition, while an empty circle indicates the Sr composition. As shown in FIG. 8, in a region where the lower electrode thickness is not less than 100 nm, the film thickness dependency of the composition of the ferroelectric thin film on the lower electrode is not seen, whereas in a region where the lower electrode thickness is less than 100 nm, the diffusion of the Bi metal atoms in the lower electrode Pt is saturated, whereby the Bi composition in the SBT increases with the decrease of the film thickness of the lower electrode. In other words, in the case where the lower electrode thickness varies in the capacitor element with a thickness of not more than 100 nm, the SBT composition also varies. As a result, the polarity characteristic of the capacitor element varies. It should be noted that “Standardized by Ta=2” regarding the vertical axis in FIG. 8 means that the composition ratio is re-calculated so that the elemental proportion of Ta2 in SBT (SrBi2Ta2O9) becomes 2. It should be noted that “Standardized by Ta=2” regarding the vertical axis in FIG. 4, which will be described later, means the same.
Further, to prevent the diffusion of metal atoms composing a ferroelectric thin film, a method whereby a metal oxide layer is arranged on a lower electrode (JP5-226715A) is disclosed. However, this makes the electrode structure complex and therefore raises a problem in the practical application.